Iron type golf club head

ABSTRACT

An iron golf club head achieving improved performance via a specific Effective Stiffness Area ratio between the boundary of the iron golf club head and the face of the iron golf club head is disclosed. More specifically, the present invention discloses a golf club head wherein an iron golf club head having a variable face profile into specific iron golf club head chassis resulting in an Effective Stiffness Area Ratio of between about 4.5 to 5.5 and a Coefficient of Restitution value as a function of the above Effective Stiffness Area Ratio.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/065,310, filed on Aug. 13, 2020, the disclosureof which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to an improved iron type golfclub head having a specific ratio of Effective Stiffness Area betweenthe boundary of the iron golf club head and the face of the iron golfclub head as well as achieving an improved Coefficient of Restitution.More specifically, the present invention relates to incorporating aniron golf club having a variable face profile into specific iron golfclub head chassis resulting in an Effective Stiffness Area Ratio ofbetween about 4.5 to 5.5 and a Coefficient of Restitution value as afunction of the above Effective Stiffness Area Ratio.

BACKGROUND OF THE INVENTION

Iron golf club heads, similar to their counterpart the wood golf clubheads, have benefitted immensely from the technological advancement thatthe industry has experienced. Although visually, the modern daymetalwood looks more different from the persimmon woods when comparingthe modern day hollow irons to the traditional mashie or niblick, thetechnological advancements are probably just game changing.

U.S. Pat. No. 4,826,172 to Antonious illustrates one of the earlieradvancements in iron golf club head design and technology by teaching aperimeter weighted iron-type golf club head with a recessed or cavityback and a peripheral mass having an improved weight configurationformed of weight members within the cavity. The shift of the mass of thegolf club head to the perimeter to create a cavity back allows a golfclub head to be more forgiving on off center shots.

U.S. Pat. No. 5,362,047 to Shaw et al. illustrates another early exampleof improving the performance of an iron type golf club head wherein thethickness of the face-piece increasing from the toe to the heel of thehead in at least one iron of the set and increasing from the heel to thetoe in at least one other iron of the set. Varying the thickness of theface-piece may further improve the performance of a golf club head byreducing material from portions of the striking face that does notrequire it.

U.S. Pat. No. 5,971,868 to Kosmatka illustrates another early attempt tocreate a contoured back surface of a golf club shown having a surfacewith varying thicknesses such that regions of the face which experiencean approximately similar load due to application of a certain force havean approximately similar thickness to result in a golf club face inwhich the stress is approximately uniform upon application of a similarforce.

U.S. Pat. No. 6,200,228 to Takeda provides a golf club with a cavityformed on the rear surface thereof and a back member securely fittedinto the cavity, with the body closely contacted each other. Thisbacking allows the increase to the strength of the head that can beimproved to enable the thickness of the face to be made thinner.

Most recently, U.S. Pat. No. 7,169,057 to Wood et al. discloses an irongolf club head having a hollow interior formed from two differentcasings, wherein the thickness and hardness of the first casing varybetween the long and short iron golf club head.

As the technology improves in the design of iron type golf club heads togradually develop away from traditional muscle back constructionstowards more cavity back and hollow cavity type constructions asillustrated by the patents above, and as striking faces of iron typegolf club heads become thinner; an opportunity arises to further improveupon the performance of the striking face of such types of golf clubheads.

Hence, based on the above, there exists a need in the art to develop animproved iron type golf club head that can leverage the new modern dayclub head chassis design. More specifically, there is tremendousopportunity to design a golf club head having a variable thicknessstriking face that is a function of the modern day club head chassis,creating an optimal performing golf club head.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is an iron golf club head comprisingof a frontal portion having a striking face defining a face height(H_(Face)), the striking face further comprising of a substantiallyplanar frontal surface and a non-planar rear surface. The non-planarrear surface of the striking face forming a VFT profile, the VFT profilefurther comprising a thickened central portion, a transition portion,and a thinned perimeter portion. The thinned perimeter portion defininga face thickness (T_(Face)) and the thickened central portion defining aVFT height (H_(VFT)) and a VFT thickness (T_(VFT)). The iron golf clubhead also comprising of a body portion attached to a rear of the frontalportion. The iron golf club head defining a boundary height(H_(Boundary)) and a boundary thickness (T_(Boundary)), the iron golfclub head having an ESA Ratio of between about 4.5 and 5.5, the ESARatio defined as

${{{ESA}\mspace{14mu}{Ratio}} = \frac{\left( {H_{Face}*T_{Face}} \right) + \left( {{0.5}*\left( {T_{VFT} - T_{Face}} \right)*H_{VFT}} \right.}{{0.5}*T_{Boundary}*H_{Boundary}}},$

andthe golf club head has a COR defined by the relationship below;

COR≥0.0196(ESA Ratio)+0.7211.

In another aspect of the present invention is an iron golf club headcomprising of a frontal portion having a striking face defining a faceheight (H_(Face)), the striking face further comprising of asubstantially planar frontal surface and a non-planar rear surface. Thenon-planar rear surface of the striking face forming a VFT profile, theVFT profile further comprising a thickened central portion, a transitionportion, and a thinned perimeter portion. The thinned perimeter portiondefining a face thickness (T_(Face)) and the thickened central portiondefining a VFT height (H_(VFT)) and a VFT thickness (T_(VFT)). The irongolf club head also comprising of a body portion attached to a rear ofthe frontal portion. The iron golf club head defining a boundary height(H_(Boundary)) of between about 35 mm to about 40 mm and a boundarythickness (T_(Boundary)) of between about 18 mm to about 24 mm, the irongolf club head having an ESA Ratio of between about 4.5 and 5.5, the ESARatio defined as

${{{ESA}\mspace{14mu}{Ratio}} = \frac{\left( {H_{Face}*T_{Face}} \right) + \left( {{0.5}*\left( {T_{VFT} - T_{Face}} \right)*H_{VFT}} \right.}{{0.5}*T_{Boundary}*H_{Boundary}}},$

andthe golf club head has a COR defined by the relationship below;

COR≥0.0196(ESA Ratio)+0.7211.

In another aspect of the present invention is an iron golf club headcomprising of a frontal portion having a striking face defining a faceheight (H_(Face)), the striking face further comprising of asubstantially planar frontal surface and a non-planar rear surface. Thenon-planar rear surface of the striking face forming a VFT profile, theVFT profile further comprising a thickened central portion, a transitionportion, and a thinned perimeter portion. The thinned perimeter portiondefining a face thickness (T_(Face)) and the thickened central portiondefining a VFT height (H_(VFT)) and a VFT thickness (T_(VFT)). The irongolf club head also comprising of a body portion attached to a rear ofthe frontal portion. The iron golf club head defining a boundary height(H_(Boundary)) of between about 35 mm to about 40 mm and a boundarythickness (T_(Boundary)) of between about 18 mm to about 24 mm. The irongolf club head has an ESA_(Boundary) of between about 380 mm² to about420 mm², said ESA_(Boundary) defined as;

ESA_(Boundary)=0.5*T _(Boundary) *H _(Boundary).

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the invention will beapparent from the following description of the invention as illustratedin the accompanying drawings. The accompanying drawings, which areincorporated herein and form a part of the specification, further serveto explain the principles of the invention and to enable a personskilled in the pertinent art to make and use the invention.

FIG. 1 of the accompanying drawings shows a rear view of a golf clubhead in accordance with an exemplary embodiment of the presentinvention.

FIG. 2 of the accompanying drawings shows a rear view of a golf clubhead in accordance with an exemplary embodiment of the present inventionwith the support member removed.

FIG. 3 of the accompanying drawings shows a rear cross-sectional view ofa golf club head in accordance with an exemplary embodiment of thepresent invention.

FIG. 4 of the accompanying drawings shows a cross-sectional view of agolf club head in accordance with an exemplary embodiment of the presentinvention, taken along cross-sectional line 4-4′ shown in FIG. 3.

FIG. 5 of the accompanying drawings shows a cross-sectional view of agolf club head in accordance with an exemplary embodiment of the presentinvention, taken along cross-sectional line 5-5′ shown in FIG. 3.

FIG. 6 of the accompanying drawings shows a toe side view of a golf clubhead in accordance with an exemplary embodiment of the presentinvention.

FIG. 7 of the accompanying drawings shows a plot that illustrates arelationship between an ESA Ratio and a COR of a golf club head for thepresent invention along with prior art golf clubs.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description describes the best currentlycontemplated modes of carrying out the invention. The description is notto be taken in a limiting sense, but is made merely for the purpose ofillustrating the general principles of the invention, since the scope ofthe invention is best defined by the appended claims.

Various inventive features are described below and each can be usedindependently of one another or in combination with other features.However, any single inventive feature may not address any or all of theproblems discussed above or may only address one of the problemsdiscussed above. Further, one or more of the problems discussed abovemay not be fully addressed by any of the features described below.

FIG. 1 of the accompanying drawings shows a rear view of a golf clubhead 100 in accordance with an exemplary embodiment of the presentinvention. Golf club head 100 may generally have a toe end 102, a heelend 104, a topline 106, and a sole 108 portion of the golf club head 100that is common in an iron type golf club heads 100. In addition toshowing these common features and their terminology, FIG. 1 of theaccompanying drawings also shows a hosel 110 portion of the golf clubhead, to which a shaft is adapted to engage a shaft (not shown).

One feature worth noting here is that FIG. 1 of the accompanying drawingshows a cavity back type golf club head 100, having a rear opening 112located at a rear portion of the golf club head 100. Cavity back typegolf club heads 100, when compared to a traditional blade type golf clubhead, generally offer more forgiveness due to the shifting of the weightto the perimeter of the golf club head 100. In addition to providingmore forgiveness, cavity back type golf club heads 100 also improvesupon the standard blade type golf club head by providing an opportunityto improve the ballspeed of the golf club head 100 by creating a “facemode” that can often yield a higher Coefficient of Restitution (COR).

Although these types of cavity back iron type golf club heads have beenpreviously associated with golfers having a higher handicap due to theneed of those players for more forgiveness, the modern day golf clubhave fully embraced this cavity back technology. In fact, the modern daygolf player, irrespective of their handicap and even tour players, oftenincorporates cavity back irons into their bags, especially when it comesto long irons; due to the transformation of these clubs into anextremely playable golf club offering the benefits to all types ofplayers.

As more and more effort are devoted into finely engineering these typeof designs, these clubs have developed deeper and fuller cavities, evengoing as far as creating a completely hollowed body construction. Inthese types of performance driven platforms for iron type golf clubhead, the trend towards thinner and thinner striking faces generally arerequired to be paired with stronger and stronger materials. These typesof thinner faces in irons, combined with them being made out of strongermaterials, especially when incorporated into chassis that yield a higher“face made” such as fuller cavity backs and hollow body constructions,may generally demonstrate higher face deflection, a higher COR, allwhile accompanied by the undesirable effect of increasing stresses.

Hence it can be seen that one of the key objectives of the presentinvention is to create a high performing iron type golf club head 100that balances the need of performance by fully taking advantage of the“face mode” generated from chassis that can benefit from such benefits,all while managing the increased stress associated with such a design.

In order to accomplish the goal of reducing stresses of these thin facediron golf club heads having a large unsupported region of the face thatcreates a “face mode”, the present invention utilizes a Variable FaceThickness (VFT) profile 114 at the back surface of the striking face111. The VFT profile 114 may generally be comprised out of a thickenedcentral portion, a transition region, and a thinned perimeter. Thethickened central portion, in this current exemplary embodiment of thepresent invention, may generally have a constant thickness. The thinnedperimeter, on the other hand, although generally also has a constantthickness, may also have a taper as well. The back view of the golf clubhead 100 shown in FIG. 1 is shown as a finished product, and has asupport member 120 contacting the rear of the VFT profile 114. In orderto better illustrate the VFT profile 114, FIG. 2 of the accompanyingdrawings is provided where the support member 120 is removed.

FIG. 2 of the accompanying drawings shows a rear view of a golf clubhead 200 in accordance with an exemplary embodiment of the presentinvention with the support member (shown as 120 in FIG. 1 removed) for aclearer illustration of the VFT profile 214. At first glance, one canimmediately see that the VFT profile 214 further comprises of twosub-components that was previously discussed. More specifically, the VFTprofile 214 further comprises a thickened central portion 214 a and atransition portion 214 b.

The thinned perimeter portion 216 of the striking face 111 (shownpreviously in FIG. 1) shown in FIG. 2 can also be separated into threesub-components, such as a heel perimeter portion 216 a located near aheel 204 of a golf club head 200, toe perimeter portion 216 c locatednear a toe 202 of a golf club head 200, and a central perimeter portion216 b juxtaposed between the heel perimeter portion 216 a and the toeperimeter portion 216 c. In a preferred embodiment of the presentinvention, the entirety of the perimeter portion 216 a, 216 b, and 216 ccould be constantly tapered from the toe perimeter portion 216 c towardsthe heel perimeter portion 216 a, with the toe perimeter portion 216 cbeing the thickest and the heel perimeter portion 216 a being thethinnest. However, in an alternative embodiment of the presentinvention, this tapering need not be constant, and can only exist in thetoe perimeter portion 216 c, the central perimeter portion 216 b, theheel perimeter portion 216 a, or any combination thereof, so long as thetoe-ward most portion is thicker than the heel-ward most portion.Alternatively speaking, it can be said that the toe portion of saidperimeter portion is thicker than a heel portion of said perimeterportion. In a further alternative embodiment of the present invention,the entirety of the perimeter portion 216 a, 216 b, and 216 c can be allof the same thickness without departing from the scope and content ofthe present invention.

Hence, looking at the entirety of the striking face 211, it can be saidthat the golf club head 200 has a variable thickness profile 214 withtwo sub-components; a thickened central portion 214 a having a highestthickness, and a transition portion 214 b having a variable thicknessthat transitions from the thickened central portion 214 a to theperimeter portions 216 a, 216 b, and 216 c. The perimeter portions 216a, 216 b, and 216 c, in a preferred embodiment, may generally be thickeron the toe than the heel, with a gradual tapering throughout. In orderto accomplish this, the heel perimeter portion 216 a could be flat ortapering, the central perimeter portion 216 b may be flat or tapering,and the toe perimeter portion 216 c could be flat or tapering; or anycombination thereof could be used all without departing from the scopeand content of the present invention. Finally, in an alternativeembodiment of the present invention, as previously mentioned, theentirety of the perimeter portions 216 a, 216 b, and 216 c could beconstant also without departing from the scope and content of thepresent invention.

In order to provide a clearer view of the various dimensions of thevariable thickness profile 214 profile, a cutaway view of the golf clubhead 200 is provided wherein the rear portion of the golf club head 200is removed. FIG. 3 of the accompanying drawings shows an enlargedcutaway view of a golf club head 300 in accordance with an exemplaryembodiment of the present invention, allowing certain additionalfeatures to be highlighted.

First off, we can see that in this cutaway view of the golf club head300 shown in FIG. 3, the variable thickness profile 314 can be seen moreclearly. The thickened central portion 314 a shown in FIG. 3 allows itsdimension to be identified, as this dimension is critical to theperformance of the golf club head 300 itself. In this embodiment of thepresent invention, the thickened central portion 314 a may have a widthW_(VFT) of between about 15 mm to about 35 mm, more preferably betweenabout 20 mm to about 30 mm, and most preferably about 26 mm. Thethickened central portion 314 a may have a height H_(VFT) of betweenabout 10 mm to about 27 mm, more preferably between about 15 mm to about22 mm, and most preferably about 18 mm. FIG. 3 also shows a center point333 of the thickened central portion 314 a.

In addition to the dimension of the thickened central portion 314 a,FIG. 3 of the accompanying drawings also shows a distance H_(Face) thatcorresponds with the height of the striking face 311 at the center ofthe face. The height of the striking face 311, as represented bydistance H_(Face) may generally be between about 50 mm to about 54 mm,more preferably between about 51 mm to about 53 mm, and most preferablyabout 52 mm.

Finally, FIG. 3 of the accompanying drawings also identifies twospecific points within the thickened central portion 314 a, a locationof maximum stress 332 and a location of maximum deflection 334. It iscritical for the present invention that the thickened central portion314 a to encompass both the location of maximum stress 332 and thelocation of maximum deflection 334, as these locations generally outlinethe locations that generally experience the highest stress, and couldbenefit from the increased material thickness associated with thethickened central portion 314 a. The location of maximum deflection 334,as referred to in this embodiment of the present invention, refersgenerally to a point that point that coincides with the hotspot of theface, which coincidentally, is also the geometric center of the face ofthe golf club head 300. The location of maximum deflection 334, cangenerally be determined by making physical measurements of the geometriccenter of the face of the golf club head 300. Alternatively, thislocation can be determined via a modal analysis to try and capture thelocation of the maximum deflection based on a prediction of the actualdeflection the golf club head 300 is expected to experience upon impactwith a golf ball. The location of maximum stress 332, on the other hand,occurs generally at a location that is directly behind the impactlocation with a golf ball. This location, is generally slightly heelwardof the location of maximum deflection 334 and slightly lower on the facethan the location of the maximum deflection 334. This location isgenerally estimated to be at the central point of the scoreline in aheel to toe direction, at a location that is about 15 mm up from theleading edge of the golf club head 300. Once again, it is worthrepeating here that the thickened central portion 314 a encompassingboth the location of maximum stress 332 and the location of maximumdeflection 334 is critical to the present invention, as both of theselocations experience higher stress than the remainder of the face whenimpacting a golf ball, and the thickened central portion's 314 aincrease thickness really help with the durability of the golf club head300.

FIG. 4 of the accompanying drawings shows a cross-sectional view of agolf club head 300 shown in FIG. 3, as illustrated by cross-sectionallines 4-4′. This cross-sectional view allows the thickness of thestriking face 411 to be shown more clearly. In this view, we can seethat the striking face 411 has a substantially planar frontal surfaceand a non-planar rear surface creating the VFT profile. Morespecifically, the VFT profile, as previously discussed, is comprised outof a thickened central portion 414 a that transitions into a perimeterportion 416 c. The thickened central portion 414 a in accordance withthis exemplary embodiment of the present invention may generally have athickness T_(VFT) of between about 1.9 mm and about 2.3 mm, morepreferably between about 2.0 mm and about 2.2 mm, and most preferablyabout 2.1 mm. Since this cross-sectional view of the golf club head 400is taken along a cross-sectional line 4-4′ shown in FIG. 3 that islocated at the central portion of the golf club head 400, the perimeterportion thickness shown here is actually the central perimeter portion416 b thickness. The central perimeter portion 416 b, located both aboveand beneath the thickened central portion 414 a, may generally have athickness T_(Face) of between about 1.6 mm and about 2.0 mm, morepreferably between about 1.7 mm and about 1.9 mm, most preferably about1.8 mm. The transition region 414 b, also located above and beneath thethickened central portion 414 a, gradually tapers in thickness from thethickened central portion 414 a towards the central perimeter portion416 b thickness.

FIG. 5 of the accompanying drawings shows a cross-sectional view of agolf club head 500 taken along cross-sectional line 5-5′ shown in FIG.3. This cross-sectional line is taken along a toe portion of the golfclub head 300 at a point that yields the highest face height value.Thus, we can see that H_(Face), although previously described in FIG. 3,along this cross-sectional view, doesn't convey all the informationrequired in FIG. 3 to provide an accurate measurement because it lackeda reference point from the side view. More specifically, the face heightdistance H_(Face) is measured along the loft of the golf club head 500that is parallel to the striking face, not parallel to the ground planeas FIG. 3 seems to suggest initially. Getting an accurate definition formeasuring the face height distance H_(Face) is an important variable indetermining an Effective Boundary Stiffness Area (ESA) of the face(ESA_(Face)), which is critical to the present invention. ESA_(Face), inaccordance with the present invention, is defined by Equation (1) below:

ESA_(Face)=(H _(Face) *T _(Face))+(0.5*(T _(VFT) −T _(Face))*H_(VFT)  Eq. (1)

Based on Equation (1) above, a golf club head 500 in accordance with anexemplary embodiment of the present invention may have an ESA_(Face) ofbetween about 14 mm² to about 25 mm², more preferably between about 16mm² to about 23 mm², most preferably about 19 mm².

FIG. 6 of the accompanying drawings shows a toe side view of a golf clubhead 600 in accordance with an exemplary embodiment of the presentinvention, allowing the overall size of the golf club head 600 to beshown. FIG. 6 goes further into identifying additional dimensions andmeasurements that will be important in determining an ESA of theboundary (ESA_(Boundary)). Ultimately, a golf club in accordance withthe present invention will have a specific ESA_(Face) to ESA_(Boundary)Ratio. First and foremost, FIG. 6 of the accompanying drawings shows thegolf club having a height of the chassis, defined as H_(Boundary),measuring the top of the golf club head 600 to the bottom of the clubhead 600 in an address position. Golf club head 600 may generally have aH_(Boundary) of between about 35 mm to about 40 mm, more preferablybetween about 36 mm to about 39 mm, and most preferably between about 37mm to about 38 mm. Once the H_(Boundary) is determined, a geometriccenter of the face 634 may be determined, at least up the face, bydividing the H_(Boundary) distance by 2. Once that center of the face634 is determined, a thickness of the chassis boundary, T_(Boundary) maybe determined as measuring rearward from the center of the face 634 atan angle that is perpendicular to the angle of the striking face itself.T_(Boundary), as shown in this current exemplary embodiment maygenerally be between about 18 mm to about 24 mm, more preferably betweenabout 19 mm to about 23 mm, more preferably between about 20 mm to about22 mm.

Once the values for H_(Boundary) and T_(Boundary) have been established,an ESA_(Boundary) can be calculated based on Equation (2) below:

ESA_(Boundary)=0.5*T _(Boundary) *H _(Boundary)  Eq. (2)

Based on Equation (2) above, a golf club head 600 in accordance with anexemplary embodiment of the present invention may have an ESA_(Boundary)of between about 380 mm² to about 420 mm², more preferably between about390 mm² to about 410 mm², most preferably about 400 mm².

Now that the ESA_(Face) and the ESA_(Boundary) have been defined andestablished, one can create a ratio between the two numbers to create anESA Ratio, which denotes one of the critical features of the presentinvention describing one way to quantify that the golf club head 600will experience a face mode. The ESA Ratio is defined by Equation (3)below:

$\begin{matrix}{{{ESA}\mspace{14mu}{Ratio}} = \frac{{ESA}\mspace{14mu}{Boundary}}{{ESA}\mspace{14mu}{Face}}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$

A golf club head in accordance with the present invention may generallyhave an ESA Ratio of between about 4.5 to about 5.5, more preferablybetween about 4.7 to about 5.3, more preferably between about 4.8 andabout 5.2. Alternatively, Equation (3) can be combined with Equations(1) and (2) to create a more detailed definition of the ESA Ratio, asshown in Equation (4) below:

$\begin{matrix}{{{ESA}\mspace{14mu}{Ratio}} = \frac{\left( {H_{Face}*T_{Face}} \right) + \left( {{0.5}*\left( {T_{VFT} - T_{Face}} \right)*H_{VFT}} \right.}{{0.5}*T_{Boundary}*H_{Boundary}}} & {{Eq}.\mspace{14mu}(4)}\end{matrix}$

FIG. 7 of the accompanying drawings show a plot that illustrates the ESARatio vs. Coefficient of Restitution (COR) of several embodiments of thepresent invention as well as four different prior art clubs. In FIG. 7shown here, the ESA Ratio is shown along the X-Axis, and the COR of thegolf club head is shown along the Y-Axis. Focusing three of the priorart golf club heads, we can see these three prior art data points do nothave ESA Ratio values between 4.5 and 5.5; and have ESA Ratio values ofabout 3.4, about 4.25, and about 4.0. The other fourth prior art datapoint, while does have an ESA Ratio value between 4.5 and 5.5, does nothave achieve a high enough COR of above 0.828 for the given ESA Ratiovalue. In order to capture the region of increased performance achievedby the present invention, an equation is provided in FIG. 7, whereiny≥0.0196x+0.7211 to help further narrow the range of performance valuespreviously unachieved within the ESA Ratio values described previously.To put the above relationship into the context of the values relevant toour current conversation, Equation (5) is provided below:

COR≥0.0196(ESA Ratio)+0.7211  Eq. (5)

Combining this discussion, it can be said that the current inventivegolf club head has an ESA Ratio of between about 4.5 and about 5.5, anda COR value that is greater than 0.0196 (ESA Ratio)+0.7211.

Finally, any discussion on thin faced iron here would not be completewithout recognizing the importance of durability. Durability of a golfclub is generally related to the amount of stress it experiences uponimpact with a golf ball. Typically, in an iron type golf club head thatis made out of steel, the durability of the golf club sufferssignificantly at any number greater than about 2.2 GPa. Alternativelyspeaking, it can be said that a golf club head in accordance with thepresent invention, in order to have a stress number less than about 2.2GPa, more preferably less than about 2.1 GPa, and most preferably lessthan about 2.0 GPa.

Other than in the operating example, or unless otherwise expresslyspecified, all of the numerical ranges, amounts, values and percentagessuch as those for amounts of materials, moment of inertias, center ofgravity locations, loft, draft angles, various performance ratios, andothers in the aforementioned portions of the specification may be readas if prefaced by the word “about” even though the term “about” may notexpressly appear in the value, amount, or range. Accordingly, unlessindicated to the contrary, the numerical parameters set forth in theabove specification and attached claims are approximations that may varydepending upon the desired properties sought to be obtained by thepresent invention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Furthermore, when numerical ranges ofvarying scope are set forth herein, it is contemplated that anycombination of these values inclusive of the recited values may be used.

It should be understood, of course, that the foregoing relates toexemplary embodiments of the present invention and that modificationsmay be made without departing from the spirit and scope of the inventionas set forth in the following claims.

1. An iron golf club head comprising: a frontal portion having astriking face defining a face height (H_(Face)), said striking facefurther comprising a substantially planar frontal surface and anon-planar rear surface, said non-planar rear surface of said strikingface forming a VFT profile, said VFT profile further comprising athickened central portion, a transition portion, and a thinned perimeterportion, said thinned perimeter portion defining a face thickness(T_(Face)), said thickened central portion defining a VFT height(H_(VFT)) and a VFT thickness (T_(VFT)), a body portion attached to arear of said frontal portion, said iron golf club head defining aboundary height (H_(Boundary)) and a boundary thickness (T_(Boundary)),said iron golf club head having an ESA Ratio of between about 4.5 and5.5, said ESA Ratio defined as,${{{ESA}\mspace{14mu}{Ratio}} = \frac{0.5*T_{Boundary}*H_{Boundary}}{\left( {H_{Face}*T_{Face}} \right) + \left( {{0.5}*\left( {T_{VFT} - T_{Face}} \right)*H_{VFT}} \right.}},$and said golf club head has a COR defined by the relationship below;COR≥0.0196(ESA Ratio)+0.7211.
 2. The iron golf club head of claim 1,wherein said iron golf club head has an ESA_(Face) of between about 80mm² to about 110 mm², said ESA_(Face) defined as,ESA_(Face)=(H _(Face) *T _(Face))+(0.5*(T _(VFT) −T _(Face))*H _(VFT).3. The iron golf club head of claim 2, wherein said ESA_(Face) isbetween about 90 mm² to about 100 mm².
 4. The iron golf club head ofclaim 3, wherein said ESA_(Face) is about 95 mm².
 5. The iron golf clubhead of claim 2, wherein said iron golf club head has an ESA_(Boundary)of between about 380 mm² to about 420 mm², said ESA_(Boundary) definedas.ESA_(Boundary)=0.5*T _(Boundary) *H _(Boundary).
 6. The iron golf clubhead of claim 5, wherein said ESA_(Boundary) is between about about 390mm² to about 410 mm².
 7. The iron golf club head of claim 6, whereinsaid ESA_(Boundary) is about 400 mm².
 8. The iron golf club head ofclaim 1, wherein said thickened central portion has said VFT thickness(T_(VFT)) of between about 1.9 mm and about 2.3 mm, said VFT height(H_(VFT)) of between about 10 mm to about 27 mm, and a VFT width(W_(VFT)) of between about 15 mm to about 35 mm.
 9. The iron golf clubhead of claim 8, wherein said VFT thickness (T_(VFT)) is between about2.0 mm and about 2.2 mm, said VFT height (H_(VFT)) is between about 15mm to about 22 mm, and said VFT width (W_(VFT)) is between about 20 mmto about 30 mm.
 10. The iron golf club head of claim 9, wherein said VFTthickness (T_(VFT)) is about 2.1 mm, said VFT height (H_(VFT)) is about18 mm, and said VFT width (W_(VFT)) about 26 mm.
 11. An iron golf clubhead comprising: a frontal portion having a striking face defining aface height (H_(Face)), said striking face further comprising asubstantially planar frontal surface and a non-planar rear surface, saidnon-planar rear surface of said striking face forming a VFT profile,said VFT profile further comprising a thickened central portion, atransition portion, and a thinned perimeter portion, said thinnedperimeter portion defining a face thickness (T_(Face)), said thickenedcentral portion defining a VFT height (H_(VFT)) and a VFT thickness(T_(VFT)), a body portion attached to a rear of said frontal portion,said iron golf club head defining a boundary height (H_(Boundary)) ofbetween about 35 mm to about 40 mm and a boundary thickness(T_(Boundary)) of between about 18 mm to about 24 mm, said iron golfclub head having an ESA Ratio of between about 4.5 and 5.5, said ESARatio defined as,${{{ESA}\mspace{14mu}{Ratio}} = \frac{0.5*T_{Boundary}*H_{Boundary}}{\left( {H_{Face}*T_{Face}} \right) + \left( {{0.5}*\left( {T_{VFT} - T_{Face}} \right)*H_{VFT}} \right.}},$and said golf club head has a COR defined by the relationship below;COR≥0.0196(ESA Ratio)+0.7211.
 12. The iron golf club head of claim 11,wherein said boundary height (H_(Boundary)) is between about 36 mm toabout 39 mm and said boundary thickness (T_(Boundary)) is between about19 mm to about 23 mm.
 13. The iron golf club head of claim 12, whereinsaid boundary height (H_(Boundary)) is between about 37 mm to about 38mm and said boundary thickness (T_(Boundary)) is between about 20 mm toabout 22 mm.
 14. The iron golf club head of claim 13, wherein said irongolf club head has an ESA_(Boundary) of between about 380 mm² to about420 mm², said ESA_(Boundary) defined as.ESA_(Boundary)=0.5*T _(Boundary) *H _(Boundary).
 15. The iron golf clubhead of claim 14, wherein said ESA_(Boundary) is between about 390 mm²to about 410 mm².
 16. The iron golf club head of claim 15, wherein saidESA_(Boundary) is about 400 mm².
 17. An iron golf club head comprising:a frontal portion having a striking face defining a face height(H_(Face)), said striking face further comprising a substantially planarfrontal surface and a non-planar rear surface, said non-planar rearsurface of said striking face forming a VFT profile, said VFT profilefurther comprising a thickened central portion, a transition portion,and a thinned perimeter portion, said thinned perimeter portion defininga face thickness (T_(Face)), said thickened central portion defining aVFT height (H_(VFT)) and a VFT thickness (T_(VFT)), a body portionattached to a rear of said frontal portion, said iron golf club headdefining a boundary height (H_(Boundary)) of between about 35 mm toabout 40 mm and a boundary thickness (T_(Boundary)) of between about 18mm to about 24 mm, wherein said iron golf club head has anESA_(Boundary) of between about 380 mm² to about 420 mm², saidESA_(Boundary) defined as.ESA_(Boundary)=0.5*T _(Boundary) *H _(Boundary).
 18. The iron golf clubhead of claim 17, wherein said ESA_(Boundary) is between about about 390mm² to about 410 mm².
 19. The iron golf club head of claim 18, whereinsaid ESA_(Boundary) is about 400 mm².
 20. The iron golf club head ofclaim 19, wherein said thickened central portion has said VFT thickness(T_(VFT)) of between about 1.9 mm and about 2.3 mm, said VFT height(H_(VFT)) of between about 10 mm to about 27 mm, and a VFT width(W_(VFT)) of between about 15 mm to about 35 mm.